Simple File Input And Output

Transcription

1 Simple File Input And Output You will learn how to write to and read from text and serialized files in Java. Storing Information On Files Types of files Text files Binary files

2 Text files - Every 8 bits represents a character - e.g., 0 = 48, 1 = 49 Text Files Binary files Includes all other types of files (it s a catch all term for non-binary files) e.g., Interpret the bit pattern as a 16 bit unsigned short (You Should Now Know Why You Can Get Garbage In Text Editors) ÿøÿà JFIF h Z ÿû C #%$""!&+7/&)4)!"0A149;>>>%.DIC<H7=>;ÿÛ C ;("(;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;ÿà ù " ÿä ÿä µ!1aqa"q2 #B±ÁRÑð$3br %&'()*456789:CDEFGHIJSTUVWXYZcdefghijstuvwxyzƒ ˆ Š š ª²³ µ ¹ºÂÃÄÅÆÇÈÉÊÒÓÔÕÖ ØÙÚáâãäåæçèéêñòóôõö øùúÿä ÿä µ w!1aqaq"2b ±Á #3RðbrÑ$4á%ñ&'()*56789:CDEFGHIJSTUVWXYZcdefghijstuvwxyz ƒ ˆ Š š ª²³ µ ¹ºÂÃÄÅÆÇÈÉÊÒÓÔÕÖ ØÙÚâãäåæçèéêòóôõö øùúÿú? îóåzoãš^õèžxp:ðh úð( $%( ö Ò % (ëfz> v ŠJ \ÐMPûÎ ëaí Pæ ÑÚ 4 PÑü )F8 9zQŸAÍþ Pêii(ö4 ½EÍ%.3@Q@hÍE`úQš( :QžzÐ);q@ Et4ƒÉ Ž v 1sëGZNô lðohæe'j M½% QžzQÞŽ Å ŒÑE EŠJ 3GãG (Å Š _ëgjoöƒ@(v Ðœæ š 3ǽbŠ ( ŽÔ qeè ½h ëå ö (ë@é@ Š ;RÒsE -žôrhïiúzb8 Š)?CŽzCÓ-/j (Í%- ½hï@úbŽz( Œþ4QÞJ3EhQEZJ=úÑ@ÂŒæŠ(G½'ô -(v <QïF(sIGj(½é(íG Ð 4hÅ v ŠP> Ð?Z( Z( (â Š> ;u R\ÐÑIŠ( Ru üè ÍÑþy ÑIïGÒ Fi;Q@ íiš(ühsiõ ÞŠ Qô µ Ç4wÉ úqšñí ( ý(?j( aš3é@ ÑÐÑGãÖƒšÑIŒó@

3 Reading Text Input From A File File : byte stream FileReader N A char stream BufferedReader AN String Writing Text Output To A File Primitives, AN Strings, Objects 1 PrintWriter N A char stream FileWriter byte stream File : 1 By objects we of course mean references to objects

4 An Example Of Simple Input And Output Location of the online example: /home/coures/219/examples/fileio/firstexample OR public class IntegerWrapper private int num; Class IntegerWrapper public IntegerWrapper () num = (int) (Math.random() * 100); public void setnum (int newvalue) num = newvalue; public int getnum () return num;

5 Class SimpleIO import java.io.*; public class SimpleIO public static void main (String [] argv) IntegerWrapper iw1 = new IntegerWrapper (); IntegerWrapper iw2 = new IntegerWrapper (); String filename = "data.txt"; PrintWriter pw; FileWriter fw; BufferedReader br; FileReader fr; Class SimpleIO (2) try fw = new FileWriter (filename); pw = new PrintWriter (fw); System.out.println("Written to file: " + iw1.getnum()); pw.println(iw1.getnum()); System.out.println("Written to file: " + iw2.getnum()); pw.println(iw2.getnum()); fw.close();

6 Class SimpleIO (3) fr = new FileReader(filename); br = new BufferedReader(fr); System.out.println("Read from file: " + br.readline()); System.out.println("Read from file: " + br.readline()); fr.close(); Class SimpleIO (4) catch (IOException e) e.printstacktrace();

7 Reading Until The End-Of-File Is Reached String filename = "data.txt"; BufferedReader br = null; FileReader fr = null; String temp = null; try fr = new FileReader(filename); br = new BufferedReader(fr); temp = br.readline (); while (temp!= null) : : : temp = br.readline (); : : : Checking For More Specific Error Types String filename = null; BufferedReader br; FileReader fr; boolean fileerror = true; while (fileerror == true) try System.out.print("Enter name of input file: "); Scanner in = new Scanner (System.in); in.nextline (); fr = new FileReader(filename); br = new BufferedReader(fr); : : fr.close (); fileerror = false;

8 Checking For More Specific Error Types (2) catch (FileNotFoundException e) System.out.println("File called " + filename + " not in the current directory"); catch (IOException e) System.out.println("General file input error occured."); e.printstacktrace(); Writing Objects Out To File: The Hard Way Location of the example: /home/courses/219/examples/fileio/secondexample OR Each field is written out to a file individually Student object: String firstname String lastname int id This approach is awkward because: 1. It requires knowledge of all the attributes of the class. 2. If attributes are not simple types or classes which can t be directly written to file the non-writable attribute must be broken down and written to file on a field-by basis. 3. Some attributes may have to be parsed or converted. data.txt Bart Simpson

9 The Driver Class public class Driver public static void main (String [] args) final String FILENAME = "data.txt"; PrintWriter pw; FileWriter fw; BufferedReader br; FileReader fr; Student astudent = new Student("Bart", "Simpson", ); int tempnum; String templine; The Driver Class (2) try fw = new FileWriter (FILENAME); pw = new PrintWriter (fw); pw.println(astudent.getfirstname()); pw.println(astudent.getlastname()); pw.println(astudent.getid()); fw.close(); fr = new FileReader(FILENAME); br = new BufferedReader(fr); astudent.setfirstname(br.readline()); astudent.setlastname(br.readline()); templine = br.readline(); astudent.setid(integer.parseint(templine)); fr.close(); System.out.println(aStudent);

10 The Driver Class (3) catch (FileNotFoundException e) e.printstacktrace(); catch (IOException e) e.printstacktrace(); catch (NumberFormatException e) e.printstacktrace(); Class Student public class Student private String firstname; private String lastname; private int id; public Student () firstname = "no name"; lastname = "no name"; id = -1;

11 public Student (String afirstname, String alastname, int anid) firstname = afirstname; lastname = alastname; id = anid; Class Student (2) public String getfirstname () return firstname; public String getlastname () return lastname; public int getid () return id; Class Student (3) public void setfirstname (String name) firstname = name; public void setlastname (String name) lastname = name; public void setid (int anid) id = anid; public String tostring () String s = new String (); s = s + "First name: " + firstname + "\n" + "Last name: " + lastname + "\n" + "ID No: " + id + "\n"; return s;

12 Writing Objects Out To File: A Better Way Location of the example: /home/courses/219/examples/fileio/thirdexample OR Write all the data for the class all at once Student object: String firstname String lastname int id Object is serialized (given a serial number) on the (output) stream data.txt Bart Simpson Objects of a class can be serialized when the class implements the Serializable interface The Driver Class public class Driver public static void main (String [] args) final String FILENAME = "data.txt"; try // Write object to file. ObjectOutputStream out = new ObjectOutputStream (new FileOutputStream(FILENAME)); Student astudent = new Student("Bart", "Simpson", ); out.writeobject(astudent); out.close(); astudent = null;

13 The Driver Class (2) ObjectInputStream in = new ObjectInputStream (new FileInputStream(FILENAME)); astudent = (Student) in.readobject(); System.out.println(aStudent); catch (FileNotFoundException e) e.printstacktrace(); catch (IOException e) e.printstacktrace(); catch (ClassNotFoundException e) e.printstacktrace(); The Student Class: Key Difference public class Student implements Serializable private String firstname; private String lastname; private int id; public Student () firstname = "no name"; lastname = "no name"; id = -1; t7

14 Slide 26 t7 constructor should call set methods tamj, 4/11/2011

15 Note: The Data File For Serialized Objects Is In Binary Form í?sr? Student ±" Ìk?I?idL? firstnamet?ljava/lang/string;l?lastnameq?~?xp? Simpson Note: Many Container Classes Are Serializable Serializable Containers: -ArrayList - LinkedList -Vector - : The effect of having a serializable container class is that the entire container can be serialized

16 Classes That Don t Implement The Serializable Interface 1. The contents of the class (data) are confidential. 2. The contents of the class is meaningful only while the program runs. You Should Now Know How to write to files with Java classes FileWriter PrintWriter How to reading text information from files with Java classes FileReader BufferedReader How objects can be written to file in a serializable form.